Fig 3.1 Post content
Reversed Carnot Cycle
As name indicates the cycle having reverse direction that is anticlockwise direction as shown in figure below:
Fig 3.2 Reversed Carnot Cycle
Fig shows T-H diagram and block diagram of reversed Carnot cycle. In T-H diagram consist of two isothermal process ( 1 -2 & 3 - 4) and two isentropic processes(2 - 3 & 4 - 1) as above.
Carnot cycle is ideal cycle which is used for comparison with actual cycle. It is imaginary cycle can not be created we discuss in limitation points why we can not used in actually.Practically, the reversed Carnot cycle cannot be used for refrigeration purpose as the isentropic process requires very high speed operation, whereas the isothermal process requires very low speed operation.
Fig 3.3 P-V and T-S diagram of Carnot cycle
Processes in Carnot cycle
Process 1-2 :Absorption of heat by the working fluid from refrigerator at constant low temperature TL during isothermal expansion.
Process 2-3 :Isentropic compression of the working fluid with the aid of external work. The temperature of the fluid rises from TL to TH.
Process 3-4 :Isothermal compression of the working fluid during which heat is rejected at constant high temperature TH.
Process 4-1 :Isentropic expansion of the working fluid. The temperature of the working fluid falls from TH to TL.

Fig 3.3 COP of Carnot Cycle


Limitations of Carnot cycle :
1. Carnot cycle is an idealization and it suffers from several practical limitations.
2. One of the main difficulties with Carnot cycle employing a gas is the difficulty of achieving isothermal compression and expansion during processes 2 - 3 and 4 - 1 respectively.
3. For a gas to have heat transfer isothermally, it is essential to carry out work transfer from or to the system when heat is transferred to the system (process 4 - 1) or from the system (process 2 - 3). This is difficult 
to achieve in practice especially when we are dealing with high speed turbines or compressors. 
4. High speed turbines or compressors will be operating closer to adiabatic condition than isothermal condition, so it is not possible to achieve isothermal compression and expansion. 
5. So to achieve adiabatic compression and expansion machine (Turbine or compressor) should operate at high speed while during isothermal compression and expansion machine should operate at low speed. This is practically impossible. 
6. In addition, the volumetric refrigeration capacity of the Carnot system is very small so amount of gas to be handled by system is large leading to large compressor displacement, which gives rise to large frictional effects. 
7. All actual processes are irreversible, hence completely reversible cycles are idealizations only and cannot be used in practice. 
8. This cycle is used as a reference or a standard to compare the actual cycles. (i.e. to check how good is our actual cycle with the best possible cycle).